Heretofore, in large-sized boilers for thermal power plants, a large apparatus for treating an exhaust gas, such as a denitrating apparatus is installed. FIG. 3 illustrates a structure in the vicinity of a denitrating device installed in a duct for an exhaust gas generated from such a large-sized boiler (combustion apparatus). In this apparatus, inlet duct 1 through which exhaust gas G is flowed in and catalyst duct 6 in which denitrating catalyst layer 4 is disposed are connected through bend portion 2; and baffle structure 3 in which plural baffle plates 3A are arranged in the direction of the cross section of the duct is disposed at the inlet of catalyst duct 6. The arrows in FIG. 3 show the direction of the flow of a gas. In this apparatus, exhaust gas G reaches, through inlet duct 1, to bend portion 2 formed at a midway of a duct for an exhaust gas, changes there its flow direction by 90°, flows into catalyst layer 4 disposed in catalyst duct 6, and then reaches to outlet duct 5 after subjected to a denitration. In the structure as described above, however, there exists such a problem as the distribution of the gas flow rate at the inlet of catalyst device 6 becomes nonuniform as shown in FIG. 3.
As the result, it is desired or requested that (1) an exhaust gas flows into catalyst layer 4 as uniformly as possible and (2) the flow of an exhaust gas which flows into catalyst layer 4 and the flow of the exhaust gas in the catalyst layer 4 become the same as much as possible, namely, the pattern of the flow passages of an exhaust gas immediately prior to flowing into catalyst layer 4 agrees with the pattern of the flow passages of the exhaust gas, formed in the catalyst layer 4. These desires or requests are produced or made based on the following reasons:
That is, (1) the reason why the flowing of an exhaust gas into catalyst layer 4 is requested to be as uniform as possible is to efficiently use the area in which the exhaust gas contacts with the catalyst in the catalyst layer 4. If the flowing of the exhaust gas into the catalyst layer was not uniform, specific portions of the catalyst in the catalyst layer 4 contact with a large amount of the exhaust gas and thus the performances of the catalyst decrease as a whole. Besides, there exists such a problem as the portions of the catalyst which contact with a large amount of the exhaust gas remarkably deteriorate and thus the life of the catalyst becomes short as a whole.
Next, (2) the reason why the flow of an exhaust gas flowing into catalyst layer 4 is requested to agree with the flow of the exhaust gas in the catalyst layer 4 as much as possible is that, for instance, when dusts such as fly ashes and unburnt matters are contained in the exhaust gas, the exhaust gas sometimes obliquely flows into the exhaust gas flow passages formed among catalysts (catalyst plates) in the catalyst layer and thus (a) the dusts directly collide against the catalyst plates to wear their surfaces and (b) the dusts adhere to and deposit on the front portions of the catalyst layer to impede the flow of the exhaust gas into the flow passages.
In order to resolve the problems described in (1) and (2) above, a method in which baffle structure 3 such as a baffle lattice is disposed at the inlet of catalyst device 6 is proposed (Japanese Patent No. 2637119).